1. Field of the Invention
This invention relates to an image forming apparatus having a transfer means for transferring images formed on an image carrying member to a transfer material (such as a paper sheet).
More specifically, this invention relates to a color-image forming apparatus of the type in which several image components of different colors are successively transferred to the same transfer material (such as a paper sheet), one superimposed on the other.
2. Description of the Related Art
Various types of so-called color-image forming apparatuses have been proposed which are equipped with a plurality of image forming sections where toner images of different colors are formed and transferred to the same transfer material, one superimposed on the other. Of these, most commonly used are color copying machines adopting the multi-color electrophotographic system.
FIG. 5 is a partial sectional view of a conventional color copying machine, which will be briefly described below.
The conventional color copying machine shown includes four image forming sections Pa, Pb, Pc and Pd, which are arranged side by side in the apparatus body. These image forming sections Pa, Pb, Pc and Pd are equipped with respective dedicated image carrying members, which, in this example, consist of electrophotographic photosensitive drums 101a, 101b, 101c and 101d.
Respectively provided around the photosensitive drums 101a, 101b, 101c and 101d are latent image forming sections 102a, 102b, 102c and 102d, developing sections 103a, 103b, 103c and 103d, transfer-discharge sections 104a, 104b, 104c and 104d, and cleaning sections 105a, 105b, 105c and 105d.
With this construction, a latent image consisting of the yellow color component of the original is first formed by the latent-image forming section 2a on the photosensitive drum 1a of the first image forming section Pa. The latent image is made visible by means of a developer which contains yellow toner and which is provided in the developing section 103a. This yellow-toner image is transferred to a transfer material 6 by the transfer-discharge section 104a.
While the yellow image is thus being transferred to the transfer material, a latent image consisting of the magenta color component of the original is formed by latent-image forming section 102b on photosensitive drum 101b by the second image forming section Pb, which is followed by the formation of a magenta-toner image by means of a magenta toner provided in the developing section 103b. When the image transfer in the first image forming section Pa has been completed, the transfer material 6 is fed to the transfer-discharge section 104b. The magenta-toner image is then transferred to a predetermined position on the transfer material 6.
In the same manner, a cyan and a black image are formed in the third and fourth image forming sections Pc and Pd, and the cyan and black color components of the original are transferred to a predetermined position on the same transfer material. When these image forming processes have been completed, the images on the transfer material are fixed thereon in a fixing section 107, thereby yielding a multi-color image. After the completion of the image-transfer operation, residual toner is removed from the photosensitive drums 101a, 101b, 101c and 101d by means of the cleaning sections 105a, 105b, 105c and 105d, respectively, thus getting the drums ready for the next latent-image formation.
In this image forming apparatus, which has the above-described construction, the transfer material 6 is fed from the right to the left (as seen in FIG. 5) by means of a feeding belt 108, successively passing the respective transfer sections 104a, 104b, 104c and 104d of the image forming sections Pa, Pb, Pc and Pd.
The material of a feeding belt used in an image forming apparatus such as a color electrophotographic copying machine having a construction as described above is generally determined from the viewpoint of the ease with which it can be formed into a belt, its durability, and so on. Thus, the following two types of material have been proposed:
(1) Polyester-fibers worked into meshes forming a belt; and
(2) A thin sheet of a dielectric material such as polyethylene-terephthalate-type resin, polyimide-type resin, or urethane-type resin, which is worked into a belt like the above material (1).
However, the inventor of the present invention has found out through experiment that the material (1) is likely to involve inter-fiber dislocation since it is woven into meshes, so that the belt itself can be deformed, which results in the transmission efficiency of the belt-feeding-speed control being deteriorated. Accordingly, the correct feeding speed cannot be maintained, distorting the images formed on transfer material 6. In addition, this mesh structure does not allow the transfer material to be kept in sufficiently close contact with the feeding belt, so that uneven image transfer is likely to occur due to the vibration o: the belt and the surface irregularities thereof.
Furthermore, it is to be noted that the size of each mesh is far larger than the diameter of the toner particles. As a result, the toner on the photosensitive drum passes through the meshes of the belt except for the portion thereof which is transferred to the transfer material on the drum, and is scattered over the components on the transfer-electrode side, such as the transfer-chargers, resulting in the transfer electrodes being contaminated with toner.
In view of this, the material of type (2) is preferred. It exhibits a high tensile elasticity, and provides a satisfactory transmission efficiency in belt-drive control. Further, its volume resistivity is generally as high as 10.sup.16 .OMEGA.cm or more, a feature which proves very advantageous for effecting electrostatic adhesion between the belt and the transfer material. As will be appreciated, this material is free from the problems experienced with the material of the above type (1).
However, since it exhibits a high volume resistivity, using this material in a case where image transfer is repeated several times, as in a color electrophotographic apparatus, results in the feeding belt being charged through image transfer, and, as the transfer is repeated, changes occur in the transfer current.
In this regard, Japanese Patent Laid-Open No. 60-57364 discloses a method of measuring the charge amount in a feeding belt. According to this method, which is to be applied to a monochrome copying machine, the surface potential of the feeding belt is measured at positions in front of and behind the transfer charger, thus controlling the transfer charger. With this method, however, the influence of the transfer current on the photosensitive drum cannot be measured, so that the transfer current cannot be controlled accurately.
Apart from this, Japanese Patent Laid-Open No. 57-99675 discloses a method according to which the surface potential of the photosensitive member (of a monochrome copying machine) is measured, thereby making it possible to control the transfer-charge amount. This method, however, does not measure the change in the surface potential in front of and behind the transfer means, which means the transfer-charge amount cannot be accurately controlled by this method.